Midwest Weekly Highlights - February 1-7, 2005

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Mild Start to February

The first week of February was generally mild and quiet across the Midwest.  Precipitation the first five days of the month was confined to the far southern portions of the region (Figure 1).  The dry weather allowed rivers and streams in the southern half of the Midwest to continue to fall after reaching flood levels due to the copious January precipitation. A low pressure system moving along the Gulf Coast with a trough extending northward on February 2nd  brought rain to an area from southern Missouri through southern Illinois, Indiana, and Kentucky.  Additional rain came on February 6-7 as a weak cold front dropped south through the Midwest.  Precipitation for the week was above normal from western Missouri and Iowa northeast through Wisconsin and northern Michigan, largely as a result of the late week cold front. (Figure 2).  Normal precipitation for the first week of February only ranges from around one inch along the Ohio River to less than 0.25 inch in northwestern Minnesota.  Actual precipitation amounts for the week ranged from 0.75 to 1.00 inch in southern and western Missouri to about 0.35 inch in northern Wisconsin and Michigan (Figure 3).

Temperatures this week were well above normal, with departures as much as 21F above normal in the northern Midwest, but only 3F to 6F above normal in the far southern Midwest (Figure 4).  Maximum temperatures on February 5 reached the 50s as far north as Lake Superior and northern Michigan.  The increasingly mild weather combined with clear skies rapidly ate away at the snow cover over the central Midwest that resulted from the storm on January 29.  (Figure 5) is a sequence of visible satellite photos showing the shrinking snow cover especially over the central Midwest during the period from February 1 to 5.  At the end of the week snow on the ground was confined to an area north of a line from St. Joseph, MO to Chicago, IL to Detroit, MI.

High Pressure Brings Quiet Weather, Poor Air Quality

For the first half of the week high pressure lay sprawled from the northern Rockies through the Midwest (Figure 6).  Light winds associated with the high combined with overnight temperature inversions over the Midwest led to worsening air quality through the fours days of  the month.  Figure 7 shows the sounding depicting the temperature profile over Detroit, MI on the morning of February 3.  The temperature at ground level was 10F, while at 5,000 feet the temperature was 29F.  The strong  inversion prevented the air near the surface from mixing with air aloft and thus dispersing the pollutants.  Air quality was especially poor around major metropolitan areas such as St. Louis,  Milwaukee, Chicago and Detroit (Figure 8, USEPA).  However,  a large portion of the Midwest experienced poor air quality.  Minnesota, Iowa, Wisconsin,  Illinois, Indiana, and Ohio posted air quality alerts.  All of these states reported an Air Quality Index (AQI) values ranging from 110 to 150, indicating unhealthy air for citizens with diminished or sensitive respiratory function.  Minnepolis-St. Paul recorded an AQI of 155 during the week, the highest in 25 years.  Air quality began to improve on February 4 as southwest winds increased ahead of a cold front pushing into the northern Midwest.